1. Field of the Invention
The present invention relates to monitoring devices for programmable controllers which monitor the programmable controller executing a program written so as to include steps provided for respective operation units of equipment to be controlled and a transition condition between the steps.
2. Description of the Related Art
Conventionally, ladder logic (ladder language), sequential function chart (SFC language), function block diagram (FBD language), etc. are known as programming languages for programmable controllers that control equipment to be controlled such as a machine tool, as specified in the IEC 61131-3 standard. In the SFC language, the flow of an individual control process is written like a flowchart. In recent years, the SFC language has been increasingly used as programs written in the SFC language are very clear.
However, programs in the SFC language are written on the assumption that steps provided for respective operation units of equipment to be controlled are sequentially executed from the very first step. Accordingly, if the equipment stops due to an abnormality while the programmable controller is executing a program written in the SFC language, it is difficult to resume the control process from the step where the abnormality occurred, when the equipment is restarted after a process of handling the abnormality is performed.
In view of this problem, the applicant proposes a programmable controller that allows equipment to be controlled to be appropriately restarted even if the equipment stops during execution of a program written in the SFC language (see Japanese Patent Application Publication No. 2008-97079 (JP 2008-97079 A)).
In the programmable controller described in JP 2008-97079 A, however, a ladder program written in the ladder language is assigned to each transition that shows a transition condition between steps, and this ladder program is configured by combining a mid-start flag that is set when the equipment is restarted with a starting condition for the step immediately before each transition, etc. Programming for this programmable controller therefore involves a large amount of work. Moreover, when performing a process of handling an abnormality, the operator sometimes performs an individual operation, namely an operation of individually operating the equipment by the operation unit. In such a case, the state of the equipment is changed by the individual operation. It is therefore particularly difficult to resume the continuous operation from an appropriate step when the equipment is restarted.